1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor device with a heterojunction bipolar transistor (HBT) in which the cutoff frequency is restrained from lowering.
2. Description of the Prior Art
Conventionally, silicon (Si) bipolar transistors whose emitter-base junctions are homojunctions have been widely used. In recent years, Si-based heterojunction bipolar transistors (HBTs) have been developed to improve the performance. Examples of such HBTs are disclosed in the Japanese Patent Publication Nos. 2-106937 and 1-289163.
The conventional HBTs have developed for the reason of the dc common-emitter current gain h.sub.FE. In detail, when a material with an energy-band gap or forbidden-band width narrower than that of silicon is used as a base, which is termed a "narrow-gap base", the gain h.sub.FE becomes higher due to its higher emitter injection efficiency than that of the conventional homojunction bipolar transistor.
Also, with the conventional homojunction bipolar transistors, in general, to realize a high speed operation of a bipolar transistor, the base width is narrow and the base doping concentration is high for reducing the base resistance. In the case, there is a disadvantage that the gain h.sub.FE decreases due to the high base doping concentration.
On the other hand, with the conventional HBTs, even if the base doping concentration is high, the gain h.sub.FE does not decrease because of the energy band structure, i.e., the band gap difference between the emitter and base of the HBTs. As a result, the gain h.sub.FE becomes higher than that of the conventional homojunction bipolar transistors in high speed applications.
A mixed crystal or alloy of silicon and germanium (Ge), i.e., SiGe, has been widely researched as a material with a narrower forbidden-band width than that of Si. A conventional semiconductor device containing an HBT with an SiGe narrow-gap base is shown in FIG. 1, which is disclosed in the Japanese Patent Publication No. 3-44937 (February, 1991).
As shown in FIG. 1, a patterned insulator film 102 for isolation is formed on the top face of an n-type Si substrate 101 and a collector electrode 109 made of aluminum (Al) is formed on the bottom face of the substrate 101. A p-type base region 113 of an npn-type HBT is selectively formed in the substrate 101 adjacent to the top face thereof. A Ge-diffusion region 110 is selectively formed inside the substrate 101 adjacent to the base region 113. The portion of the substrate 101 except for the base region 113 acts as a collector of the HBT.
A base electrode 108 made of Al is formed on the insulator film 102 to be contacted with the base region 113 through a contact hole of the film 102.
An emitter region 106 made of n-type Si is selectively formed on the insulator film 102 to be contacted with the base region 113 through another contact hole of the film 102. An emitter electrode 107 made of Al is selectively formed on the emitter region 106.
The p-type SiGe base region 113 and the diffusion region 110 are formed by diffusion of Ge. Specifically, Ge atoms are selectively diffused into the n-type Si substrate 101 from the top face thereof in solid-state or liquid-state. The Ge atoms thus diffused are distributed in the substrate 101 according to the dopant concentration profile shown in FIG. 2. The Ge content gently and monotonically decreases as depth x from the top face of the substrate 101 increases.
The dopant concentration of the p-SiGe base region 113 is also shown in FIG. 2, where the concentration decreases monotonically as depth x increases. The dopant concentration decreases more abruptly than the Ge content.
In the case that a Si-Ge base region is epitaxially grown on a single-crystal Si substrate, the base region different in composition from the substrate is directly grown on the substrate. Therefore, the Ge content abruptly changes at the interface or heterojunction of the base region and the substrate, and as a result, crystal defects readily arises at the interface or heterojunction.
With the conventional semiconductor device shown in FIGS. 1 and 2, however, the Ge content gently changes at the interface of the p-SiGe base region 113 and the n-Si substrate 101. As a result, the crystal defects are effectively restrained.
The conventional HBT with the narrow-gap base shown in FIGS. 1 and 2 has the following problems:
A first problem is the mobility reduction of the minority carrier in the base region 113 due to the backward drifting electric-field, resulting in reduction of the cutoff frequency f.sub.T.
In detail, when the Ge content at the base-emitter junction is higher than that at the base-collector junction in the base region 113, as shown in FIG. 2, for example, 15% at the base-emitter junction and 5% at the base-collector junction, the band gap difference between the base-emitter and base-collector junctions is about 75 meV and the backward drifting electric-field is about 15 to 20 kV/cm. Due to this electric-field, the cutoff frequency f.sub.T decreases by 10 GHz.
To avoid the backward drifting electric-field stated above, another conventional HBT shown in FIGS. 3 and 4 was developed, in which the Ge content at the base-emitter junction is lower than that at the base-collector junction in the base region. This device is disclosed in IEDM Technical Digest, 1990, pp13-16, written by G. L. Platton, et al.
FIG. 3 shows the dopant concentration of the HBT of G. L. Platton, et al. along the line perpendicular to the top face of the semiconductor substrate. In FIG. 3, the reference numerals 31, 33, 30, 23 and 22 show an emitter electrode made of n-type polysilicon, an n-type emitter region, a p-type intrinsic base region, an n-type collector region, and an n.sup.+ -type buried region, respectively.
As shown in FIG. 3, the Ge content linearly increases from 0% at the interface of the emitter electrode 31 and the emitter region 33 to 10% at the interface of the base region 30 and the collector region 23, i.e., the base-collector junction. The Ge content linearly and abruptly decreases from 10% at the base-collector junction to 0% in the collector region 23.
With the conventional HBT shown in FIGS. 3 and 4, there is a disadvantage that the p-type dopant such as boron (B) doped into the p-type base region 30 are diffused again toward the n-type collector region 23 during a heat-treatment process or processes in the fabrication process steps thereof, so that the base-collector junction shifts toward the collector region 23. As a result, the cutoff frequency f.sub.T of the HBT deteriorates abruptly due to increase in width of the base region 30 at higher collector current levels, which is a second problem.
The second problem is caused from the following reason:
As described above, the p-type impurity atoms doped into the base region 30 are diffused and redistributed thermally in the vicinity of the base-collector heterojunction, so that the dopant concentration profile thereof shown by the solid line A in FIG. 3 changes to that shown by the broken line B. As a result, the base-collector junction shifts toward the collector region 23.
Therefore, an additional base region whose width is .DELTA.W is produced by the thermal diffusion of the impurity atoms, so that a resultant width W.sub.1 of the base region 30 thus expanded is equal to the sum of the initial width W.sub.0 of the original base region 30 grown by molecular beam epitaxy (MBE) and the width .DELTA.W of the additional base region, i.e., W.sub.1 =W.sub.0 +.DELTA.W.
FIG. 4 shows an energy-band diagram of the HBT shown in FIG. 3, where E.sub.c and E.sub.V represent energies at the bottom of the conduction band and at the top of the valence band, respectively, and E.sub.F represents the Fermi energy.
Due to the shift of the base-collector junction, a parasitic energy barriers EB are generated in the conduction and valence bands in the vicinity of the base-collector junction, as shown in FIG. 4. The energy-band gap in the resultant base region 30 expanded is not such a narrow gap as SiGe and is similar to that of Si.
The parasitic energy barriers EB reduce the number of the carriers injected from the base region 30 into the collector region 23, so that the collector current decreases. Also, the transit time of the carriers travelling across the base regions 30 increases as the height of the barriers EB increases. As a result, the cutoff frequency f.sub.T showing the high-frequency characteristic of the HBT lowers at higher collector current levels.
The relationship between such the parasitic energy barriers and the impurity redistribution in the base region is disclosed in IEDM Technical Digest, 1989, pp 639-642, written by E. J. Prinz, et al.
The parasitic energy barriers EB are caused not only by the metallurgical shift (described above) of the base-collector junction due to diffusion of the impurity atoms doped into the base region but also by the change of the effective base width generated at higher collector current levels, which is termed the "base push-out".